1. Technical Field
The present invention relates to wireless communications and, more particularly, to circuitry for filtering and amplifying signals.
2. Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards, including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc., communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of a plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via a public switch telephone network (PSTN), via the Internet, and/or via some other wide area network.
Each wireless communication device includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier stage. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier stage amplifies the RF signals prior to transmission via an antenna.
Typically, the data modulation stage is implemented on a baseband processor chip, while the intermediate frequency (IF) stages and power amplifier stage are implemented on a separate radio processor chip. Historically, radio integrated circuits have been designed using bi-polar circuitry, allowing for large signal swings and linear transmitter component behavior. Therefore, many legacy baseband processors employ analog interfaces that communicate analog signals to and from the radio processor.
In conventional designs of radio receivers, and especially of integrated circuit radio receivers with a large baseband frequency filter, direct current (DC) offset is a known problem. With multiple gain stages in a receiver front end, the DC offset can saturate the linear range of the gain stages. A significant problem that exists is that a typical high-pass transfer function for a feedback based amplifier with a resistor in the feedback loop as well as at the input is that of the high-pass corner frequency changing as a function variable gain amplifier (VGA) gain levels within a high-pass VGA. The greater the change in VGA gain, the greater the change in the high pass corner frequency. Typically, the corner frequency of the high-pass filter amplifier increases with the increase in gain. The undesirable consequence is that more of a DC or low frequency signal is amplified by the gain stages resulting in lower filtering or blocking of such signal components as the corner frequency increases. Additionally, overall bandwidth is lowered when the high-pass frequency corner changes.
In many systems, the final analog bandwidth is sampled and converted to digital form with an analog-to-digital converter. A subsequent digital filter may then be used to recover the desired analog signal bandwidth and impart the required DC offset cancellation. The problem with this approach, however, is that there is considerable gain in the analog processing path such that cumulative offsets will saturate the intermediate stages in the filter chain. Thus, even in a system that employs a form of DC offset cancellation, the drift of the high frequency corner as described may result in lower frequency components not being cancelled which would have been cancelled without frequency drift. Thus, these DC offset components that are not cancelled may be subsequently amplified at each gain stage. In this situation, the digital form signal produced to the baseband processor or digital filters is substantially saturated thereby limiting the ability of the baseband processor to provide digital compensation. Thus, it is desirable to provide adequate compensation in the analog processing path such that cumulative DC offset does not saturate downstream amplifiers or analog-to-digital converters.
One approach to solve the problem of the high-pass corner movement is to use a very slow DC offset cancellation system that has a very low high-pass corner. This approach, however, has the drawback that there is significant delay while an offset cancellation loop settles. Moreover, other approaches that may provide reasonable linearity and variable gain are low bandwidth systems. Thus, there is a need for a system or design that provides a high-corner that provides filtering at the desired low frequency without movement due to increases in and is independent of amplifier gain and provides linearity over a wide frequency bandwidth. For example, it is desirable to provide linearity over a 20-30 MHz bandwidth with linearity over a gain of 60 dB in a variable gain amplifier.
Moreover, in an integrated circuit utilizing MOSFETs as configured and biased to operate as resistors, or, alternatively, in an integrated circuit using MOSFET switches to switch resistances in and out of connectivity to set the gain, process variations in generating bias signals for the MOSFET switch or resistor result in non-linear response due to square-law voltage-current relationships within MOSFET devices as is known by one of average skill in the art. It is desirable, therefore, to avoid the effects of non-linearity known to exist for such MOSFET resistors and MOSFET switches and to provide adequate filtering of low frequency components without frequency drift that results from gain level changes.